This invention relates to objectives for telephoto lenses and more particularly, to telephoto objectives suitable for close-up photography.
Most photographic objectives are focused by moving their constitutent components as a single unit. However, it is possible to focus an objective by moving either its front component or its rear component. With selective use of proper power distribution, moving the front component to focus offers the advantage of reducing the range of movement for the moving component from the range of movement required when the objective as a whole is moved. However, the front component itself must be properly corrected for aberrations. Otherwise, the overall aberrations would tend to vary substantially during focusing. On the other hand, focusing with the rear member has the advantage of taking place in a space where the height of incidence of on-axial rays is small and facilitates minimization of the weight of the movable focusing component. Moreover, focusing with the rear member prevents variation of the overall length of the objective during focusing and results in little shift of the center of gravity.
The telephoto objective of U.S. Pat. No. 4,045,128 is focused by moving a sub-group within the objective rear group. However, as this sub-group is axially moved to focus down on shorter object distances, the focal length of the entire optical system becomes shorter. For this reason, the image magnification at the same object distance is lowered as compared with the other focusing provision made at the entire system or the front member. It is known to provide a telephoto objective with its rear group having two sub-groups arranged to be axially moved in opposite direction to each other to effect focusing down, as disclosed in U.S. Pat. No. 3,854,797. It is also known to provide a telephoto objective in which either one of the front group or the front sub-group of the rear group is made movable for focusing as disclosed in U.S. Pat. No. 4,113,357. However, these conventional telephoto objectives when assessed from the point of view of a valuable increase in the image magnification, are incapable of satisfying a sufficient picture performance. If focusing is effected down to unduly short object distances, there will be a high possiblity of calling for a great increase in the range of movement of the focusing member, an objectionable lowering of the picture taking quality, or a large increase in the bulk and size of the entire lens system.
We see again that the optical system of the type capable of close-up photography is made axially movable usually as a whole toward the front to effect focusing. In this case, the amount of forward movement of the optical system is related to the image magnification by the following formula (1) EQU .beta.=-x'/f (1)
and the relationship between the image magnification and the object distance may be expressed by the following formula (2) EQU g.sub.1 =-(1-1/.beta.)f (2)
where
.beta.: the image magnification; PA1 x': the amount of forward movement from the position for an infinitely distant object; PA1 f: the focal length of the entire optical system; and PA1 g.sub.1 : the distance from the front principal plane of the optical system to an object being photographed.
Therefore, to achieve a desirable increase in the image magnification for the same amount of forward movement, the use of smaller values of f is advantageous. On the other hand, for the same image magnification, the object distance may become greater if f is increased. In practice, for example, when photographing a moving object such as an insect or where a great degree of freedom is needed in selecting the position of the main light source, the so-called working distance should be increased. With a 50 mm lens being used for taking a close-up photograph of .beta.=-0.5x, the required values of forward movement and working distance are 25 mm and 150 mm respectively. With a 200 mm lens, they become 100 mm and 600 mm respectively. However, the increase in the working distance imposes the requirement of imparting a large amount of forward movement to the lens. This makes it necessary to elongate the axial length of the usual helicoid focus control, or to use long bellows. Moreover, as the image mangification is varied by readjusting the position of the lens, the resulting large amount of forward movement adversely affects the ease and speed of handling, and the way the camera is held must be adapted to cope with a large shift in the center of gravity.